Synergy is a $20 software product from UK company Symless, which allows you to use just one mouse and keyboard to control two or more computers. Any combination of Windows, Macintosh, or Linux machines can be used.
With Synergy, you still need a separate monitor for each machine, but you can just roll your mouse seamlessly from one to another. Wherever the mouse pointer is, you can click and type. I find it very convenient for using Logic Pro X on my Mac at the same time as NetVSTHost on a Windows PC.
Click on the title above to go to the Symless web site. Ignore their videos, where they they try far too hard to sell you the Synergy product. Just buy it. It works fine, and it's well worth the $20.